Even so, the dog remains highly sought after, especially in the US where it was the fourth most popular pure breed in 2015.

Breeding out mutations not the answer

Dr Niels Pedersen, from the Centre for Companion Animal Health at the University of California, Davis, who led the new study published in the journal Canine Genetics And Epidemiology, said: "The English bulldog has reached the point where popularity can no longer excuse the health problems that the average bulldog endures in its often brief lifetime.

"More people seemed to be enamoured with its appearance than concerned about its health. Improving health through genetic manipulations presumes that enough diversity still exists to improve the breed from within, and if not, to add diversity by out-crossing to other breeds. We found that little genetic 'wiggle room' still exists in the breed to make additional genetic changes."

Changes to the genome of the dog and its ancestors had taken place over hundreds of years, but had gained pace over the last few decades, he pointed out.

Breeding out all the mutations linked to ill-health was not the answer, as this would further reduce genetic diversity.

"We would also question whether further modifications, such as rapidly introducing new rare coat colours, making the body smaller and more compact and adding more wrinkles in the coat, could improve the bulldog's already fragile genetic diversity," Dr Pedersen added.

The scientists looked at 102 British bulldogs - 87 from the US and 15 from other countries. Their DNA was compared with that of another 37 British bulldogs brought to the US Davis Veterinary Clinical Services for health problems.

Breeders in Switzerland have started to out-cross the British bulldog with the Olde English Bulldogge - paradoxically, an American breed.

The resulting Continental Bulldog is an attempt to improve the animal's health, said the researchers. But many breeders were opposed to any reduction of the British Bulldog's purity.

'Huge rise in demand'

Several bulldog breeds exist today. Confusingly, given the existence of the recently created Olde English Bulldogge breed, the original Old English Bulldog is now extinct. It was a compact, muscular animal with vice-like jaws, bred for bull-baiting.

When bull-baiting was banned in 1835, the Old English Bulldog was crossed with the pug to produce a shorter, wider dog with a characteristically flat snout.

Sean Wensley, senior vet at the leading veterinary charity PDSA, said: "This study raises serious issues about the health and welfare of English Bulldogs. On a daily basis PDSA vets see the devastating health consequences of selectively breeding for features, such as flattened faces and folded skin, which some people find appealing but which cause some of the most serious and preventable health problems for dogs, including the English Bulldog.

"Flat-nosed breeds typically suffer from marked breathing problems which can have a serious and lifelong impact on their quality of life. Bulldogs also suffer from eye diseases and spinal problems, and the majority are unable to give birth naturally, with around 90% being delivered by Caesarian section.

"We have recently seen a huge rise in demand for breeds such as the English Bulldog, French Bulldog and Pug, which is only serving to exacerbate the problem, and we would urge prospective owners to consider the health problems experienced by these breeds and choose an alternative, healthier, breed or crossbreed instead.

"This study highlights that there may be difficulties in adequately addressing the health problems of English bulldogs without out-crossing to other breeds, and all approaches should now be fully considered to ensure the quality of life of dogs is prioritised over appearance, popularity or profit."